Shapechanger. The Gerathon can use its action to polymorph into a Huge giant constrictor snake, or back into its true form. Its statistics, other than its AC, are the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.

Multiattack. The Gerathon makes 3 attacks with its bite.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 8d6 piercing damage plus 5d8 poison damage. 

Lash of the Serpent. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 9d10 slashing damage plus 4d12 poison damage.

The Gerathon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Gerathon regains spent legendary actions at the start of its turn.

Bite. The Gerathon makes one attack with its bite.

Move. The Gerathon moves up to half its speed.